1. Field of the Invention
The present invention relates to a method for forming a pattern with both logic-type and memory-type circuit, and more particularly provides the application of masks in double-exposure process.
2. Description of the Prior Art
A mask for photolithography, replicated during printing, is a fundamental tool in the photoresist process. It consists of a transparent plate covered with a patterned film of opaque material. The plate is generally made of fused quartz. The advantages of quartz is that it is transparent to deep Ultra Violet (.ltoreq.365 nm) and has a very low thermal expansion coefficient. The opaque material is typically a very thin (.ltoreq.100 nm) film of chrome, covered with an anti-reflective coating (ARC), such as chrome oxide (.ltoreq.20 nm), to suppress interferences at the wafer surface. A mask containing both transparent and opaque areas is used to print the pattern onto the photoresist coated on wafer. The light beams traverse the transparent areas of the mask to the surface of the photoresist and are resisted by the opaque areas.
In general, there are two kinds of circuit patterns that could be contained on the mask, and those include memory-type and logic-type. Therein, the memory-type circuit has a very high density and the logic-type circuit has a lower density with a very small critical dimension (CD). The critical dimension (CD) is the size of the most important (usually the smallest) line width, spacing, or contact hole.
It is necessary to apply an individual strategy of masks to the different kinds of patterns. Usually, memory-type circuit pattern is applied by the half-tone mask or the Levenson mask while the logic-type circuit pattern is applied by the phase-edge mask. For the E-DRAM pattern containing the two types of circuit, in general, it is very difficult to get a good resolution. Therefore, new strategies of masks for forming the pattern of E-DRAM need to be developed. They should not only improve the resolution of the image, but also should be economically fabricated to follow the trend of future development of E-DRAM.